1. Field of the Invention
This invention relates to marine hydraulic steering systems and hydraulic lock valves used in conjunction therewith.
2. Description of Related Art
Hydraulic steering systems are preferred on small pleasure and fishing boats instead of the more usual cable steering systems. A problem is encountered however in conventional hydraulic steering systems when they are used on high power boats in particular. Such systems normally include a reversible rotary pump which is mechanically coupled to the steering wheel. Hydraulic lines extend from this manual pump to a hydraulic cylinder attached to the outboard motor or inboard/outboard motor. However a high force is exerted on the cylinder, and consequently on the steering wheel, by the rudder or engine torque. Accordingly, the boater must maintain a hold on the wheel to keep the boat on course.
For these reasons, it is conventional to provide hydraulic steering systems for high powered boats with lock valves. Conventional lock valves are often included in the same housing as the pump connected to the steering wheel, but they could be separate and located in different places such as the back of the boat near the motor. Conventionally these valves include two ports which are connected to the pump and two ports which are connected to the cylinder for two line hydraulic systems. In such systems the two ports on the pump alternate as intake and discharge ports depending upon the direction the steering wheel is turned. The lock valve usually includes an internal spool valve and two check valves or poppet valves. When the wheel is rotated, pressurized fluid from the pump enters one of the ports on the lock valve. The pressurized fluid forces open one of the check valves or poppet valves, thus allowing the fluid to discharge from one of the ports towards the hydraulic cylinder. Hydraulic fluid returning from the other side of the cylinder must reach the intake side of the pump. Normally this flow is blocked by the other check valve. However, the spool valve is shifted by the pressurized fluid from the pump and pushes against the second check valve, opening a return passageway for fluid.
However, there is an inherent problem encountered with conventional hydraulic steering systems including such lock valves. The steering wheels are initially unresponsive and must be turned a considerable amount, often 47.degree.-82.degree. or more depending upon the type of system and equipment, before the rudders or engines respond. Boaters find this a great inconvenience as it does not provide the immediate turning response required for high powered boats such as bass boats. In an effort to do away with the deadband, boaters often resort to hydraulic steering systems without a lock valve at all or to cable steering systems. They prefer the inconvenience of holding the wheel to maintain course, rather than have to deal with an unresponsive steering system with large degrees of deadband.
This problem has been recognized for some time and numerous attempts have been made to minimize the deadband in such hydraulic steering systems. It was thought that the volume of fluid required to move the spool was the source of the problem. Thus much of the effort focused on reducing the movement of the spool valve. Attempts were also made to reduce the spool diameter to cut the volume of fluid flow. Also the check valves were moved closer together so the spool only had to move very small amounts to unseat the check valves. However this did not reduce the deadband significantly and also required close machining tolerances and therefore made the valves expensive.
Another problem encountered with previous lock valves is chatter which occurs when the helm is steered in the same direction the load is acting. The spool in the valve oscillates back and forth, contacting the balls of the check valves and opening and closing the balls under load. The resulting pressure spikes and impact of the spool on the balls and spool stops can cause a disconcertingly loud chattering noise. Steering performance is also diminished.
Earlier U.S. Pat. No. 5,349,878, of which I am a co-inventor, provides some solutions to these problems. However some problems remained. First there is a possibility of hydraulic fluid leaking from the pressurized port in the lock valve circumferentially about the valve spool to the return port. Second, previous arrangements have unequal fluid pressure acting on different portions of the spool at times, resulting in potential spool sticking or uneven operation. Also, previous designs are often prone to fluid leakage past the lands of the valve spool. Also, leakage of fluid about the spool can lead to wheel drift.